1. Field of the Invention
This invention relates to mode couplers, and more particularly, to wavelength selective mode couplers.
2. Description of the Prior Art
A mode coupler is a device which causes energy to be transferred between fundamental modes of a waveguide. Mode couplers are useful in applications such as optical filtering and optical switching, and can be constructed from optical waveguides by bending the waveguide in predetermined patterns. One such mode coupler is disclosed in the article "Fiber-optic modal coupler using periodic microbending", by J. N. Blake et al. in Optics Letters, Vol. 11, No. 3, March 1986, at pages 177-179. In the disclosed mode coupler, an optical fiber, which propagates the LP01 and LP11 modes, is compressed between corrugated surfaces such that a periodic "microbending" is induced in the fiber, resulting in coupling between the two modes. Since this mode coupler shows only a slight dependence on the wavelength of the propagating energy, its use in filtering applications is limited. Further, these mode couplers cannot be cascaded easily.
This cascading problem can best be explained with reference to FIGS. 1 and 2. FIG. 1 shows a schematic representation of the cross-section of modes LP01 and LP11 as they propagate in a typical waveguide. The arrows indicate the direction of the electric fields present in each mode and the corrugation is assumed to be in the Y direction. The prior art waveguides, such as the mode coupler in the above-mentioned article, can only transfer energy from the LP01(a) mode to the LP11(a) mode, but not to the LP11(c) mode. Similarly, the mode coupler can only transfer energy from the LP01(b) mode to the LP11(b) mode, but not to the LP11(d) mode. Thus, after mode coupling, the electric field of the LP11 mode is polarized identically to the electric field of the LP01 mode. With this in mind, note that FIG. 2 shows two prior art mode coupling devices 202 and 203, coupled together at junction 201. According to well-known linear systems theory, the response of the entire device of FIG. 2 should be the product of the responses of the two separate waveguides 202 and 203. However, because the microbending is sinusoidal in the Y direction only, as shown, waveguides 202 and 203 must be aligned exactly so that the microbending in both waveguides is in the same direction. If the direction of the microbending of one of the waveguides is rotated slightly from the direction of the microbending of the other one, the cascaded pair of devices will not operate correctly. This alignment problem, caused by the fact that the bending is in only the Y direction, is extremely difficult to accomplish with current technology, and represents a drawback to the use of microbending to induce mode coupling. It is an aspect of the invention to eliminate the need for this exact alignment. It is another aspect of the invention to provide a mode coupler which can be utilized easily in filtering applications.